1. Field of the Invention
The present invention relates to a drive method of a light-emitting display panel using, for example, organic electroluminescence (EL) elements as light-emitting elements and to a display device using the light-emitting display panel, and more particularly, to a control technology for controlling the luminance of the light-emitting display panel when it is lit.
2. Description of the Related Art
Attention is given to an organic EL display unit as a display unit replacing a liquid crystal display unit because the organic EL display unit can reduce power consumption, can display an image of high quality and further can be reduced in thickness. This is because the efficiency and life of the organic EL display unit have been improved to a practically usable level by using an organic compound promising good light emitting characteristics for the light-emitting layers of EL elements used in the EL display unit.
There has been proposed a passive matrix drive system as a drive method of a display panel in which the EL elements are disposed. FIG. 3 shows the passive matrix drive system and an example of the display panel whose light emission is controlled by the passive matrix drive system. Two drive methods, that is, a cathode line scan/anode line drive method and an anode line scan/cathode line drive method are available as a drive method of the EL elements in the passive matrix drive system. FIG. 3 shows the arrangement of the former cathode line scan/anode line drive method.
That is, the display panel 1 is arranged such that anode lines A1-An are disposed longitudinally as n-pieces of drive lines, whereas cathode lines B1-Bm are disposed laterally as m-pieces of scan lines, and organic EL elements OEL shown by the symbol of a diode are disposed at the intersections (n×m positions in total) of the respective lines. Then, the respective EL elements acting as light-emitting elements constituting pixels are disposed in a lattice shape, and one ends thereof (anode terminals of the EL elements) are connected to the anode lines and the other ends thereof (cathode terminals of the EL elements) are connected to the cathode lines in correspondence to the intersections of the vertical anode lines A1-An and the horizontal cathode lines B1-Bm. Further, the anode lines are connected to an anode line drive circuit 2, and the cathode lines are connected a cathode line scan circuit 3 so as to be driven respectively.
The anode line drive circuit 2 is provided with drive switches SX1-SXn in correspondence to the respective anode lines A1-An, and these drive switches Sx1-Sxn act to supply either the currents from constant current circuits I1-In or a ground potential to the anode lines corresponding to the respective anode lines A1-An. Accordingly, when the drive switches SX1-SXn are connected to the constant current circuit, they act to supply the currents from the constant current circuits I1-In to the respective EL elements disposed in correspondence to the cathode scan lines.
In contrast, the cathode line scan circuit 3 is provided with scan switches SY1-SYm in correspondence to the respective cathode scan lines B1-Bm, and any of the cathode scan lines B1-Bm being scanned is selectively connected to the ground potential acting as a reference potential. With this arrangement, the currents from the constant current circuits I1-In are supplied to the respective EL elements, which are disposed in correspondence to the cathode scan lines, through the drive switches Sx1-Sxn, thereby the EL elements are emitted.
In this case, a reverse bias current VM, which has a value near to the forward voltages of the EL elements being driven for light emission, is applied to the cathode lines other than the cathode line being scanned through the scan switches SY1-SYm, thereby the EL elements, which are not lit, are prevented from erroneously emitting light (crosstalk emission).
Note that it is possible to use a voltage source such as a constant voltage circuit, and the like in place of the constant current circuit I1-In. However, the constant current circuit shown in FIG. 3 is ordinarily used because of the reasons that the voltage/luminance characteristics of the EL elements are unstable to a temperature change while the current/luminance characteristics thereof are stable to the temperature change and that there is a possibility that the EL elements are deteriorated by an excessive current, and the like.
The anode line drive circuit 2 and the cathode line scan circuit 3 are connected to a light emission control circuit 4 including a CPU through control buses. The scan switches SY1-SYm and the drive switches SX1-SXn are manipulated based on the image signals of an image to be displayed. With this arrangement, the constant current circuits I1-In are appropriately connected to desired anode lines while setting the cathode scan lines to the ground potential at a predetermined cycle based on the image signals. Accordingly, the respective EL light-emitting elements selectively emit light, thereby the image is reproduced on the display panel 1 based on the image signals.
A DC output (output voltage=VH) from a drive voltage source 6 composed of, for example, a voltage increasing type DC-DC converter is supplied to the respective constant current circuits I1-In of the anode line drive circuit 2. With this arrangement, the constant currents created by the constant current circuits I1-In having received the output voltage VH from the drive voltage source 6 are supplied to the respective EL elements disposed in correspondence to the anode scan lines.
The reverse bias voltage VM used to prevent the crosstalk light emission of the EL elements is ordinarily generated by series regulating the output voltage VH because the value of the voltage VM is relatively near to the value of the output voltage VH and the current consumption of the reverse bias voltage VM is smaller than that the current consumption of the output voltage VH. It is considered that the employment of the above arrangement is advantageous from the view point of the number of parts and power consumption.
A reverse bias voltage creation circuit 5 arranged simply as shown in FIG. 3 can be preferably employed as a series regulating circuit. The reverse bias voltage creation circuit 5 receives the output voltage VH from the drive voltage source 6 at a series circuit composed of a constant voltage diode ZD1 and a resistor R1, and the reverse bias voltage VM is obtained from the terminal voltage of a resistor R1 via an output resistor R2. That is, the reverse bias voltage VM is obtained by subtracting a constant voltage determined by the constant voltage diode ZD1 from the output voltage VH.
It is known that the organic EL element described above has diode characteristics including a predetermined electric capacitance (parasitic capacitance) due to the laminated structure thereof. FIG. 4 shows an equivalent circuit of the organic EL element, and the equivalent circuit can be shown by a light-emitting element E having diode characteristics and a parasitic capacitance CP connected in parallel to the light-emitting element E.
Accordingly, when the organic EL element is driven by the constant current, the anode voltage waveform of the EL element rises up slowly because the constant current circuit is a high impedance output circuit in operation principle as shown in FIG. 5. That is, in FIG. 5, a vertical axis shows the anode voltage V of the element, and a lateral axis shows an elapsed time t.
The rising-up curve of the anode voltage V is changed by various conditions such as the lighting/non-lighting condition of the EL elements when they were scanned last time, the lighting/non-lighting condition of adjacent EL elements, and the like. In particular, the organic EL element emits light when the anode voltage thereof reaches a relatively high light emitting threshold voltage Vth. Accordingly, the luminance of the display panel is substantially dropped inevitably because the organic EL element emits light at a time t1 and thereafter (it does not emit light before the time t1 ).
To cope with this problem, there has been proposed a drive method of connecting a constant voltage source to an EL element when the EL element is driven for light emission and providing a precharge period for instantly charging the parasitic capacitance Cp of the EL element. There is a cathode reset method as a typical drive method of executing the precharge as disclosed in, for example, Japanese Patent Application Laid-Open No. 9-232074.
FIG. 6 explains a cathode reset method making use of the reverse bias voltage VM created in the drive circuit arranged as described above as the precharge voltage of a light-emitting element. A cathode reset operation is executed by driving the drive switches SX1-SXn in the anode line drive circuit 2 or the scan switches SY1-SYm in the cathode line scan circuit 3 in response to the control signal from the light emission control circuit 4 shown in FIG. 3.
Note that FIGS. 6A-6D show, for example, from a state in which an EL element E11 connected to the first anode drive line A1 is driven for light emission to a state in which an EL element E12 connected to the first anode drive line A1 likewise is driven for light emission in the next scan. In FIG. 6, EL elements being driven for light emission are shown by a symbol mark of a diode and the other EL elements are shown by a symbol mark of a capacitor.
FIG. 6A shows a state before the cathode reset operation is executed in which the cathode scan line B1 is scanned and the EL element E11 is emitted. The EL element E12 is emitted in the next scan. However, before the EL element E12 is emitted, the anode drive line A1 and all the cathode scan lines B1-Bm are reset to the ground potential as shown in FIG. 6B to thereby discharge all the charges. This is executed by connecting the respective scan switches SY1-SYm shown in FIG. 3 to the ground as well as by connecting the drive switch SX1 connected to the first drive line A1 to the ground.
Next, the cathode scan line B2 is scanned to emit the EL element E12. That is, the cathode scan line B2 is connected to the ground, and the reverse bias voltage VM is applied to the cathode scan lines other than the cathode scan line B2. Note that, at this time, the drive switch SX1 is isolated from the ground and connected to the constant current circuit I1.
Since the charges of the parasitic capacitances of the respective EL elements have been discharged in the above reset operation shown in FIG. 6B, the parasitic capacitances of the EL elements other than the EL element E12 which is emitted next are charged with the reverse bias voltage VM in a reverse direction as shown by arrows in FIG. 6C at the moment, and the currents charged to these parasitic capacitances flow to the EL element E12 which is emitted next through the anode drive line A1 and charges (precharges) the parasitic capacitance of the EL element E12. At this time, the constant current circuit I1 connected to the drive line A1 is basically the high impedance circuit as described above and does not influence the behavior of the charged current.
In this case, when it is assumed that 64 EL elements, for example, are disposed to the anode drive line A1 and that the above reverse bias voltage VM is 9 V, the potential V (A1) of the anode drive line A1 instantly rises up to the potential shown in the following equation 1 due to the above charge operation because the wiring impedance in the display panel is too small to ignore. For example, in a display, panel having an outside dimension set to 100 mm×25 mm (256×64 dots), this operation is completed in about 1 μsec.V(A1)=(VM×63+0V×1)/64=8.86V  [Formula 1]
Thereafter, the EL element E12 is caused to instantly emit light as shown in FIG. 6D by the drive current that flows from the constant current circuit I1 to the anode drive line A1. As described above, the cathode reset method acts to instantly rises up the forward voltage of the EL element that is drive for light emission next making use of the parasitic capacitance CP of the EL element that essentially obstructs the drive thereof and the reverse bias voltage VM for preventing the crosstalk light emission.
Incidentally, the display device that is driven for light emission by the above arrangement is provided with a gradation control function and a dimmer control function for controlling the display luminance thereof. The former gradation control function mainly controls the luminance of each EL element for each dot. Further, the latter dimmer control function mainly controls the overall luminance of the display panel uniformly. When the dimmer control function is employed in a device for automobile use, it is used to control overall luminance in accordance with the outside light in the daytime and at night.
FIG. 7A exemplifies a case in which the gradation and dimmer control are executed by controlling gradation in 4 stages dimmer in 32 stages, respectively. That is, in a display device shown in FIG. 7A, the cathode reset Rs described above is executed in synchronism with a line sink Ls showing one line of display as well as the gradation and dimmer control is executed in the remaining period subsequent to the cathode reset Rs.
As shown in FIG. 7A, the light-emitting elements are lit by time division during the control period DRn in which the gradation and dimmer control is executed. Note that the upper numerals in FIG. 7A show the lighting control executed by the dimmer control and the lower numerals show the lighting control executed by the gradation control. That is, the number of stages of gradation D is set to 0-3 stages and the number of stages of dimmer L is set to 0-31 stages, and the lighting drive period of the EL elements is divided into 3×31=93. Then, currents are supplied to the EL elements from the constant current circuits as many as the number of D×L to thereby execute the gradation and dimmer control.
When, for example, the gradation is set to 3 and the dimmer is set to 31, any relevant EL elements are lit during all the periods. Further, when, for example, the gradation is set to 3 and the dimmer is set to 30 stages, the EL elements are not lit during the period shown by 31 in FIG. 7A. Further, when, for example, the gradation is set to 2 and the dimmer is set to 31, the EL elements are lit during only the periods 1 and 2 in the lower numerals. With this operation, the light emitting times of the EL elements being lit and displayed are controlled, thereby the light emitting luminance of the display panel is controlled.
Incidentally, when the EL elements start to be lit, the voltage near to the reverse bias voltage VM is precharged to the parasitic capacitances of the EL elements by the above cathode reset. Accordingly, when a case in which the dimmer is set to, for example, 1 is taken into consideration, a problem is arisen in that the display panel is driven and lit with a certain degree of light emission luminance LX. This is because a voltage, which permits the EL elements to be sufficiently emitted, has been precharged to the EL elements by the cathode reset operation as shown in FIG. 7B regardless of the requirement for essentially controlling the EL elements to reduce the light emission luminance thereof to a very low level.
FIG. 8 is a graph showing an example of the light emission luminance, in which a lateral axis shows the number of stages of dimmer and a vertical axis shows light emission luminance (cd=candela). Note that the characteristics shown in FIG. 8 show a case in which the gradation is set to 3. Even if the dimmer is set to 1 as shown as characteristics “a” in FIG. 8, the EL elements emit light with luminance of about 10 cd. Then, when the dimmer is set to 1, the EL elements are not lit.
Accordingly, the luminance is greatly changed between a case in which the EL elements are not lit and a case in which the dimmer is set to 1. Further, a rate of change of luminance is relatively small between a case in which the dimmer is set to 1 and a case in which the dimmer is set to 31 to cause the EL elements to be lit with maximum luminance. This drawback is caused by that the reverse bias voltage VM is set (fixed) to an approximately constant value regardless of that the dimmer is set to various values.